Notrump opening bids show a balanced hand and can be made with a weak five-card major suit or a five-card minor suit.To open 1NT if possible you should try to ensure that you can control all your suits by having an honor card  in each. You should also have an honor card of Queen or higher in all of the suits and your hand should not contain a void, a singleton or a worthless doubleton.
Distribution of your hand can be in either of the three following patterns
5, 3, 3, 2
or
4, 3, 3, 3
or
4, 4, 3, 2
If your cards are in either of these three patterns your hand is called "balanced" or "flat"

Opening with a 1NT Bid · 12-14 HCP · a balanced hand · No more than 1 doubleton (2 cards)* · Without a 5 card strong major suit (2 honour cards) · Your hand may contain a 5 card minor suit

*Note

A doubleton when in a NoTrump contract should contain an honor card with the rank of Queen or higher. If the doubleton has two cards neither of which is a Q or higher then it is referred to as a "worthless doubleton" because effectively there are no stoppers in that suit